1. Field of the Invention
The present invention relates to a technique advantageously applicable to a thermal head having printing heating elements arranged in a row and driver circuits provided in the vicinity of the heating elements to drive the heating elements so as to make the heating elements develop heat, and also advantageously applicable to a line thermal printer having such a thermal head.
2. Description of the Related Art
A head the structure of which is shown in FIG. 5 is known as a thermal head of a line thermal printer for use in a cash register or the like. This thermal head 2 has heating elements 22 and driver integrated circuits (driver ICs) 23 for driving the heating elements 22 to develop heat. The heating elements 22 and the driver ICs 23 are arranged on an insulating substrate 21. A wiring film 24 for externally supplying power and signals to the driver ICs 23 is connected to the thermal head 2. The driver ICs 23 are embedded in a resin or the like. The thus-constructed thermal head 2 is placed so as to face a platen roller 4. Printing is performed on a printing sheet 8 by causing the heating elements 22 to develop heat while pressing the printing sheet 8 against the heating elements 22 by the platen roller 4 and intermittently feeding the printing sheet 8 by the platen roller 4.
In a line thermal printer using the above-described thermal head 2, the direction of transport of the printing sheet 8 relative to the thermal head 2 is such that the printing sheet 8 is passed through the nip between the heating elements 22 and the platen roller 4 from the side where a driver IC 23 embedding portion 23a exists to the side where the driver IC 23 embedding portion 23a does not exist.
In the thermal head using the above-described sheet transport method, however, it is not possible for the printing sheet 8 to advance to the heating elements 22 parallel to the head surface, i.e., the surface on which the heating elements 22 are arranged, since the driver IC 23 embedding portion 23a protrudes on the upstream side of the heating elements 22 with respect to the direction of transport of the printing sheet 8. As shown in FIG. 5, the sheet 8 is supplied obliquely downwardly to the printing position and is curved on the upstream side of the heating elements 22.
When printing is performed by using a comparatively thin and sufficiently flexible sheet, the printing sheet, fed obliquely downwardly as described above, becomes substantially parallel to the thermal head when passing the heating elements, so that printing can be normally performed. However, if printing is performed by using a thick paper or a hard printing sheet, the printing sheet fed obliquely downwardly cannot sufficiently closely contact the heating elements, resulting in occurrence of a printing failure such as a patchy condition.
In view of the above-described circumstances, the present invention has been made to overcome the drawbacks in the conventional art. It is therefore an object of the present invention to provide a thermal printer in which, even when a thick or hard printing sheet is used, the sheet and a thermal head are parallel to each other about the position on the heating elements, and which can therefore perform normal. Another object is to provide a thermal head suitable for use in the thermal printer.
To achieve these objects, according to the present invention, there is provided a line thermal printer comprising a thermal head having printing heating elements arranged in a row, driver circuits for driving the heating elements, and an embedding portion in which the driver circuits are embedded portion in which the driver circuit are embedded protruding beyond a surface on which the heating elements are arranged, characterized in that the direction of transport of a printing sheet at the time of printing is set such that the printing sheet is passed through a place above the heating elements from the side where the embedding portion does not exist to the side where the embedding portion exists.
In the thus-constructed printer, the driver circuit embedding portion does not obstruct feed of a printing sheet to the thermal head, and the sheet feed direction can be set such that the angle from the surface on which the heating elements are arranged is substantially zero, that is, the direction can be set substantially parallel to this surface. Therefore, even if a thick or hard printing sheet is used, it can be maintained parallel to the thermal head even downstream of the printing position in the sheet feed direction, and printing can be normally performed without a failure such as occurrence of a patchy condition.
If there is an embedding portion on the sheet entrance side of the head, the sheet insertion opening is reduced that much and there is a need to provide a sheet guide having an end located close to the nip between the platen roller and the thermal head in order to prevent a printing sheet from being caught by the embedding portion at the time of setting of the sheet. In the above-described printer, however, there is no embedding portion on the sheet entrance side, so that the sheet insertion opening can be increased. Thus, a simpler sheet guide may suffice for guiding a printing sheet to the increased sheet insertion opening.
Preferably, sheet guides for stopping sides of a printing sheet are provided on the side of the above-described thermal head where there is no embedding portion. Since there is no driver circuit embedding portion on the sheet inlet side, it is not necessary to largely extend, on the sheet inlet side, the head base on which the heating elements are arranged. Therefore, the portion for guiding a printing sheet to the heating elements may be reduced. However, a simple sheet guide may be provided on the sheet inlet side to enable a printing sheet to be easily led to the heating elements, as described above. Such a sheet guide comprises one having upper and lower members between which a sheet guide slot is formed.
Further preferably, the center of rotation of a platen roller used to transport a printing sheet while pressing the printing sheet in the vicinity of the heating elements is offset along a direction toward the embedding portion from a line extending from the heating elements perpendicularly to the surface on which the heating elements are arranged. When a printing sheet is introduced from the side where there is no embedding portion, the printing sheet is brought into contact with the embedding portion to be bent upwardly downstream of the printing position. Therefore, the possibility of occurrence of a patchy print may be considered. However, the above-described placement of the head and the platen roller ensures that any printing sheet can be maintained parallel to the thermal head even downstream of the printing position.
Further, a thermal head may be provided which has a base on which printing heating elements are arranged in a row, and on which driver circuits for driving the heating elements are formed, and an embedding portion in which the driver circuits are embedded, the embedding portion protruding beyond a surface on which the heating elements are arranged. In this thermal head, the distance between the row of the heating elements and the end of the base on the side where the embedding portion does not exist is set to 7 mm or more.
In the thus-constructed thermal head, when a printing sheet is led to the heating elements, the excess portion of the base at the side where there is no embedding portion can function as a sheet guide for introducing the printing sheet into the nip between the base and the platen roller, for example. Therefore, if this thermal head is used in a line thermal printer in which the direction of sheet transport at the time of printing is set such that a printing sheet is passed a place above the heating elements from the side where the embedding portion does not exist to the side where the embedding portion exists, the printing sheet can be led to the heating elements of the thermal head advantageously easily.
Further, a sheet guide for guiding a printing sheet from the side of the thermal head where the embedding portion does not exist to the heating elements may be provided on the side where the embedding portion does not exist, thereby achieving similar effect. Such a sheet guide comprises sheet guides for guiding sides of a printing sheet.